System apparatus and method for matching antenna impedance in a wireless communication system

ABSTRACT

Provided is a method for matching antenna impedance in a wireless communication system. The method includes determining an approximate reflection coefficient based on an input signal and an output signal of a bidirectional coupler connected to a signal path of an antenna; determining an antenna impedance matching parameter corresponding to the determined approximate reflection coefficient by using a lookup table; and performing antenna impedance matching based on the antenna impedance matching parameter.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a divisional application of U.S. application Ser. No. 15/438,099filed Feb. 21, 2017, which claims priority from Korean PatentApplication No. 10-2016-0093720, filed in the Korean IntellectualProperty Office, on Jul. 22, 2016, the entire disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

Apparatuses and methods consistent with exemplary embodiments relate tomatching antenna impedance in a wireless communication system.

BACKGROUND

In a wireless communication system, the transmission efficiency of anantenna may be affected by the input impedance of the antenna.Accordingly, in general, a transmitter performs impedance matching sothat a reflection coefficient in a transmission antenna port may beminimized. Impedance matching refers to an operation in which a loadimpedance that varies in a process of installing the antenna in aterminal or varies depending on various radio environments and the useenvironment of the terminal is matched to the characteristic impedanceso that the maximum power transfer to the antenna may be achieved.

The related art fixed-impedance matching circuit performs antennaimpedance matching in advance based on a characteristic impedance value(e.g., 50 ohm or 75 ohm) to avoid reflection losses by matchingimpedances of both sides at an in-between contact point so that all ofthe power supplied from a signal source may be transferred to the load.

The related art fixed-impedance matching circuit may be designed suchthat the antenna impedance matching network is relatively simple inconfiguration, and does not occupy the large area.

However, in the related art fixed-impedance matching circuit, itsimpedance matching range is limited, and in particular, when the loadimpedance is changed due to various reasons, the antenna impedancematching is difficult to perform. A corresponding S-parameter (orScattering-matrix) is provided by the vender. The S-parameter refers toa ratio of an output voltage to an input voltage seen at each port ofthe circuit processing a radio frequency (RF) signal, and by using theS-parameter, an RF circuit may perform impedance matching to minimizethe reflection losses. However, in the actual circuit, because it isdifficult for every terminal to extract the S-parameter, there is noeffective impedance matching scheme for the communication environmentsthat vary with respect to each terminal.

Meanwhile, in a wireless communication system, there is a death gripphenomenon in which the transmission and/or reception sensitivity of theradio waves is abruptly lowered in the situation where the loadimpedance of the terminal varies. Here, the death grip may occur, forexample, when the user's hand is in contact with the conductive frame ofthe terminal, or wraps around the antenna-embedded part of the terminal.When the death grip phenomenon occurs, the performance degradation mayoccur since the frequency shift and loss increase due to the contactbetween the user's body and the conductive frame or the antenna beingcovered by the user's hand.

There is a need for a scheme capable of preventing occurrence of thedeath grip phenomenon and improving the transmission and/or receptionperformance.

SUMMARY

One or more exemplary embodiments provide a method and apparatus capableof increasing the efficiency of transmission power of an antenna in awireless communication system.

Further, one or more exemplary embodiments provide a method and anapparatus capable of increasing the transmission power of an antennawithout a complex operation in a wireless communication system.

Moreover, one or more exemplary embodiments provide a method and anapparatus for substantially preventing a death grip phenomenon in whichthe transmission and/or reception sensitivity of the radio waves isabruptly lowered in the situation where the load impedance of theterminal varies, in a wireless communication system.

In accordance with an aspect of an exemplary embodiment, there isprovided a method for matching antenna impedance in a wirelesscommunication system. The method may include determining an approximatereflection coefficient based on an input signal and an output signal ofa bidirectional coupler connected to a signal path of an antenna;determining an antenna impedance matching parameter corresponding to thedetermined approximate reflection coefficient by using a lookup table;and performing antenna impedance matching based on the antenna impedancematching parameter.

In accordance with an aspect of another exemplary embodiment, there isprovided an apparatus for matching antenna impedance in a wirelesscommunication system. The apparatus may include a controller configuredto determine an approximate reflection coefficient based on an inputsignal and an output signal of a bidirectional coupler connected to asignal path of an antenna, and determine an antenna impedance matchingparameter corresponding to the determined approximate reflectioncoefficient by using a lookup table; and an antenna impedance matchingunit configured to perform antenna impedance matching based on theantenna impedance matching parameter.

In accordance with an aspect of still another exemplary embodiment,there is provided an apparatus for matching antenna impedance in awireless communication system. The apparatus may include a bidirectionalcoupler connected to a signal path of an antenna; and a controllerconfigured to determine an approximate reflection coefficient based onoutput power of the bidirectional coupler in a reverse path and outputpower of the bidirectional coupler in a forward path of thebidirectional coupler, and perform antenna impedance matching based onthe approximate reflection coefficient.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects will be more apparent by describingcertain exemplary embodiments with reference to the accompanyingdrawings, in which:

FIGS. 1A, 1B, and 1C are block diagrams of antenna impedance matchingdevices in a wireless communication system according to exemplaryembodiments;

FIG. 2 is a diagram illustrating a detailed structure of a bidirectionalcoupler in an antenna impedance matching device in a wirelesscommunication system according to an exemplary embodiment;

FIG. 3 is a flowchart illustrating a process of measuring an approximatereflection coefficient for lookup table (LUT)-based real-time antennaimpedance matching according to an exemplary embodiment;

FIG. 4 is a detailed flowchart illustrating a process of measuring anapproximate reflection coefficient for LUT-based real-time antennaimpedance matching according to an exemplary embodiment;

FIGS. 5 and 6 are graphs illustrating a relationship between actual loadimpedance and reflection coefficients measured according to an exemplaryembodiment;

FIG. 7 is a graph illustrating impedance matching performance as per anumber of reference impedance points to be included in an LUT forefficiency of an LUT size according to an exemplary embodiment;

FIG. 8 is a graph illustrating a matching result before impedancematching is performed, a matching result obtained by using a 23-pointLUT, and a matching result obtained through exhaustive search accordingto an exemplary embodiment;

FIGS. 9 and 10 are graphs illustrating transmission power as per loadimpedance before and after application of closed loop-antenna impedancetuning (CL-AIT) according to an exemplary embodiment;

FIG. 11 is a graph illustrating power gain between FIG. 9 (beforeapplication of an AIT LUT solution) and FIG. 10 (after application ofthe AIT LUT solution);

FIG. 12 is a graph illustrating an example of applying an antennaimpedance matching method according to another exemplary embodiment;

FIG. 13 is a block diagram of an antenna impedance matching deviceaccording to another exemplary embodiment;

FIG. 14 is a flowchart illustrating an antenna impedance matching methodaccording to another exemplary embodiment;

FIG. 15 is a diagram illustrating a fixed-point structure of an infiniteimpulse response (IIR) filter according to another exemplary embodiment;

FIGS. 16A and 16B and FIGS. 17A and 17B are graphs illustrating acorrelation result with a time-domain signal when a resource block (RB)size is 1 and 50 respectively, in long term evolution (LTE);

FIG. 18A is a graph illustrating forward and reverse real-time measureddelay values according to another exemplary embodiment;

FIG. 18B is a graph illustrating convergence of delay register settingsin a case where forward and reverse IIR filter outputs are greater thanor equal to a threshold, according to another exemplary embodiment;

FIG. 18C is a graph illustrating a change in forward and reverse IIRfilter outputs according to another exemplary embodiment; and

FIG. 19 is a graph illustrating a result obtained by measuring b4/b3according to another exemplary embodiment.

DETAILED DESCRIPTION

Hereinafter, exemplary embodiments will be described in detail withreference to the accompanying drawings. In the following description, adetailed description of known functions and configurations will beomitted, if it is determined that the description unnecessarily obscuresthe subject matter of the disclosure. The terms described below areterms defined in consideration of the functions in the disclosure, andmay vary depending on the intention of the user/operator or the customs.Therefore, the definition should be made based on the contentsthroughout the specification.

Although terms used in the disclosure are selected with general termspopularly used at present under the consideration of functions withrespect to the exemplary embodiments, the terms may vary according tothe intention of those of ordinary skill in the art, judicialprecedents, or introduction of new technology. In addition, in aspecific case, the applicant voluntarily may select terms, and in thiscase, the meaning of the terms is disclosed in a correspondingdescription part of the disclosure. Thus, the terms used in thedisclosure should be defined not by the simple names of the terms but bythe meaning of the terms and the contents throughout the disclosure.

A base station is an entity communicating with a terminal, and may referto a base station (BS), a Node B (NB), an evolved Node B (eNB), accesspoint (AP), and the like.

A terminal (or a communication terminal) is an entity communicating witha base station or another terminal, may refer to a node, a userequipment (UE), a mobile station (MS), a mobile equipment (ME), adevice, a terminal, and the like.

FIGS. 1A, 1B, and 1C are block diagrams of antenna impedance matchingapparatuses in a wireless communication system according to exemplaryembodiments.

In the wireless communication system, an antenna impedance apparatusaccording to an exemplary embodiment as shown in FIG. 1A may include anantenna impedance tuning (AIT) controller 103, a first tunable matchingnetwork (TMN) 104 and a second TMN 105. The AIT controller 103 maycontrol the TMN 104 and the second TMN 105.

The first TMN 104 represents an impedance tuner, and may efficientlyperform (or maximize) power transfer by matching impedance using a TMNparameter via a serial connection.

The second TMN 105 represents an aperture tuner, and may increase (ormaximize) the radiation efficiency in a free space by tuning theimpedance matching or resonant frequency using the TMN parameter, via aparallel connection.

The AIT controller 103 may perform impedance matching by updating atleast one TMN parameter among TMN parameters of the first TMN 104 andthe second TMN 105 to adjust a load impedance of an antenna end based ona reflection coefficient (or an approximate reflection coefficient)according to an exemplary embodiment.

In another exemplary embodiment, in the wireless communication system,an antenna impedance matching apparatus as shown in FIG. 1B may includethe first TMN 104 and the AIT controller 103 that controls the first TMN104. In other words, the AIT controller 103 may perform impedancematching by updating the TMN parameter of the first TMN 104.

In still another exemplary embodiment, in the wireless communicationsystem, an antenna impedance matching apparatus as shown in FIG. 1C mayinclude the second TMN 105 and the AIT controller 103 that controls thesecond TMN 105. In other words, the AIT controller 103 may performimpedance matching by updating the TMN parameter of the second TMN 105.

Accordingly, in exemplary embodiments, the impedance matching may beperformed in a manner that the AIT controller 103 updates (or adjusts)the TMN parameter of the first TMN 104 based on the approximatereflection coefficient, updates (or adjusts) the TMN parameter of thesecond TMN 105 based on the approximate reflection coefficient, and/orupdates (or adjusts) the TMN parameters of the first TMN 104 and thesecond TMN 105 based on the approximate reflection coefficient.

Referring to FIG. 1A, the antenna impedance matching apparatus mayinclude a power amplifier PA, a low noise amplifier LNA, a duplexer 100,a bidirectional coupler 101, a feedback controller 102, the first TMN104, the AIT controller 103 and the second TMN 105. In the antennaimpedance matching apparatus according to an exemplary embodiment, amodem and a radio frequency (RF) unit may be configured in a front endof the power amplifier PA and the low noise amplifier LNA.

The power amplifier PA may amplify a signal converted in the front endto a preset signal level, and transmit the amplified signal to theduplexer 100.

The duplexer 100 may transmit the signal received from the poweramplifier PA to the bidirectional coupler 101.

The bidirectional coupler 101 is connected with the feedback controller102 of the front end including the RF unit, and may distribute signalsto two or more ports, and/or gather signals at one port. Further, thebidirectional coupler 101 may perform signaling along a forward pathand/or a reverse path by using a feedback loop to perform measurement ofthe approximate reflection coefficient.

In other words, the bidirectional coupler 101 may transmit the signaltransmitted through the antenna, via the feedback path in the forwarddirection, or transmit a reflected wave of the transmission signal tothe feedback controller 102 in the reverse direction.

The feedback controller 102 may detect the received signal that ispropagated in the forward path signaling or reverse path signaling, andstore the detected signal in a memory (not shown). The signals stored inthe memory may be used to approximately estimate the load impedance thatis changed through time alignment.

Further, the feedback controller 102 may obtain the approximatereflection coefficient based on the received signal that is propagatedin the forward path signaling or reverse path signaling, and transmitthe obtained approximate reflection coefficient to the AIT controller103.

The AIT controller 103 may set a ratio between levels of receivedsignals on the forward path and the reverse path as an approximatereflection coefficient, and control at least one of the first TMN 104and the second TMN 105 to perform real-time antenna impedance matchingbased on the approximate reflection coefficient, by using a lookuptable. The process of setting the ratio between levels of receivedsignals as the approximate reflection coefficient will be describedusing Equations (1) to (5) below.

The AIT controller 103 may precisely search the lookup table using theapproximate reflection coefficient, and obtain a desirable TMN parametercorresponding to the approximate reflection coefficient through thesearched lookup table. The lookup table may be implemented to includeTMN parameters that may be used to provide high (or maximum) outputpower with respect to reference impedance positions that are predefinedbased on the approximate reflection coefficient.

For example, the lookup tale may be implemented as a table in which TMNparameters corresponding to respective indices of the approximatereflection coefficients are mapped thereto. The approximate reflectioncoefficients refer to reference load points (e.g., the referenceimpedance points) that are preset to correspond to a change in loadimpedance of the antenna end according to an exemplary embodiment.Therefore, when an approximate reflection coefficient is obtained basedon the changed load impedance of the antenna end, a TMN parametercorresponding to the approximate reflection coefficient may bedetermined using the lookup table. Accordingly, it is possible toperform impedance matching using the determined TMN parameter. The TMNparameter represents inductance and capacitance values of, for example,a π network or a T network.

The AIT controller 103, in an exemplary embodiment, may set a region (ora low-matching gain region) having a low-matching gain that isdetermined in advance based on a load coverage of a previously used TMN,and promptly or immediately move the load belonging to the low-matchinggain region to a high (or maximum) output voltage point instead ofmatching the load to a characteristic impedance.

The AIT controller 103, in an exemplary embodiment, may provide areference point for selecting an impedance matching gain of a TMN as pera voltage standing wave ratio (VSWR), for comparison with the currentimpedance level. As a result of the comparison, the AIT controller 103may promptly or immediately move the load belonging to the low-matchinggain region to a desirable output point (e.g., high output voltagepoint) instead of performing impedance matching. The VSWR refers to aheight ratio of a standing wave generated by reflection, and may be usedto express the reflection coefficient. VSWR has a value close to 1, whenthere is substantially no reflection, and is proportional to the amountof reflection.

The AIT controller 103 may set reference impedance by using the lookuptable, and compensate load impedance of the antenna end by using TMNparameters mapped to the reference impedance corresponding to themeasured impedance (e.g., the approximate reference coefficient).

The AIT controller 103 may set a load using a load pull tuner, andprovide a database (DB) in advance using TMN parameters to performcompensation of the set load impedance. Here, the load pull tuner isexternal equipment to generate desired antenna impedance by connectingwith a terminal instead of the antenna, and may apply the desiredimpedance to the terminal by connecting with the terminal instead of theantenna.

The MT controller 103 may control to perform real-time antenna impedancematching by activating at least one of the first TMN 104 and the secondTMN 105 using the desired TMN parameter. Here, the lookup table stores aresult of operation in a memory in advance to reduce the number ofoperations in the process, and thus facilitates the process by using thestored result.

FIG. 2 is a diagram illustrating a detailed structure of a bidirectionalcoupler in an antenna impedance matching apparatus in a wirelesscommunication system according to an exemplary embodiment.

The antenna impedance matching apparatus in FIG. 2 may include abidirectional coupler 201, a TMN 203, and an AIT controller 202. Here,the TMN 203, as shown in FIGS. 1A to 1C, may include at least one of thefirst TMN 104 and the second TMN 105. The bidirectional coupler 201corresponds to the bidirectional coupler 101 in FIGS. 1A to 1C.

In FIG. 2, a1, a2, b1, b2, b3, and b4 represent signals according toforward and reverse signal flows, respectively. Here, a1 and a2represent input signals at respective ports of the bidirectional coupler201, and b1, b2, b3, and b4 represent output signals at respective portsof the bidirectional coupler 201.

Assuming that outputs of the bidirectional coupler 201 and inputs of thelow noise amplifier LNA are properly matched, a 4-port S-parameter isrepresented as a matrix shown in Equation (1) below.

$\begin{matrix}{\begin{pmatrix}b_{1} \\b_{2} \\b_{3} \\b_{4}\end{pmatrix} = {\begin{bmatrix}S_{11} & S_{12} & S_{13} & S_{14} \\S_{21} & S_{22} & S_{23} & S_{24} \\S_{31} & S_{32} & S_{33} & S_{34} \\S_{41} & S_{42} & S_{43} & S_{44}\end{bmatrix}\begin{pmatrix}a_{1} \\a_{2} \\0 \\0\end{pmatrix}}} & (1)\end{matrix}$

In Equation (1), the S-parameter represents a ratio of output power toinput power at each port. For example, S₁₂ represents a ratio of outputpower at a first port Port1 to input power at a second port Port2.Therefore, S₁₁, S₂₂, S₃₃ and S₄₄ represent reflection coefficients atrespective ports.

Through the matrix in Equation (1), a signal b3 that corresponds to asignal transmitted through the antenna to the feedback controller 102via the forward path (e.g., along a signal path ofa1=>Port1=>Port3=>b3), and a signal b4 that corresponds to a reflectedwave of the transmission signal transmitted to the feedback controller102 via the reverse path (e.g., a signal path of a2=>Port2=>Port4=>b4)may be represented as shown in Equation (2) below.b ₃ =S ₃₁ a ₁ +S ₃₂ a ₂ =S ₃₁ a ₁ +S ₃₂(b ₂┌_(in))b ₄ =S ₄₁ a ₁ +S ₄₂ a ₂ =S ₄₁ a ₁ +S ₄₂(b ₂┌_(in))  (2)

Assuming that the bidirectional coupler 201 is symmetric such asS₃₁≈S₄₂, and has substantially no cross-talk (e.g., S₄₁=S₁₄≈0,S₃₂=S₂₃≈0), signal levels of signals b3 and b4 may be approximated asshown in Equation (3) below.b ₃ ≈S ₃₁ a ₁b ₄ ≈S ₄₂ b ₂┌_(in) S ₄₂(S ₂₁ a ₁)Γ_(in)  (3)

Therefore, a reflection coefficient at the output of the bidirectionalcoupler 201 may be approximated as shown in Equation (4) below.

$\begin{matrix}{{\Gamma_{in} \approx \frac{b_{4}}{S_{42}{S_{21}\left( {b_{3}/S_{31}} \right)}}} = {\frac{S_{31}b_{4}}{S_{42}S_{21}b_{3}} = \frac{b_{4}}{S_{21}b_{3}}}} & (4)\end{matrix}$

Herein, b4/b3 may be expressed as shown in Equation (5) below.

$\begin{matrix}{\frac{b_{4}}{b_{3}} \approx {S_{21}\Gamma_{in}}} & (5)\end{matrix}$

Because a linear gain S₂₁ of the bidirectional coupler 201 is a constantwith respect to a fixed carrier frequency, b4/b3 may be an approximatereflection coefficient as per the load change according to an exemplaryembodiment. Further, b4/b3 may be measured as a ratio of the output ofthe bidirectional coupler 201 in the reverse path to the output of thebidirectional coupler 201 in the forward path through the feedback pathas shown in Equation (6) below.

$\begin{matrix}{\frac{b_{4}}{b_{3}} = {\frac{{{s_{{fwd},{tx}}(t)}}^{2}}{{{s_{{rev},{tx}}(t)}}^{2}}\frac{\max\limits_{\tau \in D}\left( {{s_{{rev},{tx}}(t)} \otimes {s_{{rev},{rx}}\left( {t + \tau} \right)}} \right)}{\max\limits_{\tau \in D}\left( {{s_{{fwd},{tx}}(t)} \otimes {s_{{fwd},{rx}}\left( {t + \tau} \right)}} \right)}}} & (6)\end{matrix}$

In Equation (6), among the subscripts of each signal s(t), “fwd”represents passing through the forward branch of the bidirectionalcoupler 201, and “rev” represents passing through the reverse branch ofthe bidirectional coupler 201. Further, “tx” represents a transmittedcomplex baseband signal and “rx” represents a received complex basebandsignal.

As described above, the b4/b3 value calculated through the measurementmay be one-to-one mapped to the reflection coefficient r_(in). Further,the approximate reflection coefficient (e.g., b4/b3) that is one-to-onemapped to the reflection coefficient r_(in) according to an exemplaryembodiment may also be determined through the experiment results inFIGS. 5 and 6 described below. Herein, it should be noted that b4/b3 (orb4/b3 value) and a b4/b3 matrix are interchangeably used.

Therefore, the antenna impedance matching apparatus according to anexemplary embodiment may perform lookup table (LUT)-based real-timeantenna impedance matching using the approximate reflection coefficient.

FIG. 3 is a flowchart illustrating a process of measuring an approximatereflection coefficient to perform LUT-based real-time antenna impedancematching according to an exemplary embodiment.

In operation 300, a terminal is in a standby state until its operationpoint is a CL-AIT operation point or more based on a trigger condition(e.g., the operation point of the terminal satisfies the triggercondition). The terminal may determine in operation 302 whether itsoperation point is the CL-AIT operation point or more based on thetrigger condition. In other words, the CL-AIT operation point may beconstrued as a criterion (or condition) for determining whether toperform antenna impedance matching, by measuring the power of the signaltransmitted through the antenna.

In response to its operation point being the CL-AIT operation point ormore based on the trigger condition (e.g., determining whether toperform the impedance matching), the terminal may perform real-timemonitoring on the bidirectional coupler in operation 304. In operation306, the terminal may operate the above-described bidirectional couplerthrough the forward path or reverse path. In operation 308, the terminalmay detect an input/output signal that is propagated in the forward pathand/or reverse path of the bidirectional coupler. The terminal may storethe detected input/output signal in a memory. In other words, theterminal may transfer input/output signals (e.g., signalsS_(fwd,tx)(t),S_(fwd,rx)(t+τ),S_(rev,tx)(t),S_(rev,rx)(t+τ), as per theexample specified in FIG. 2 and Equation (6)) as per the coupler mode toa dedicated hardware (H/W) block (e.g., a memory and the like) throughthe feedback loop by operating the bidirectional coupler, to store theinput/output signals in the dedicated hardware block.

In operation 310, the terminal may time-align signals (e.g., signalsS_(fwd,tx)(t),S_(fwd,rx)(t+τ),S_(rev,tx)(t),S_(rev,rx)(t+τ)) as per theexample specified in FIG. 2 and Equation (6), calculate b4/b3 usingthese signals, and set the calculated b4/b3 as an approximate reflectioncoefficient. In operation 312, the terminal may search the lookup tableusing the calculated approximate reflection coefficient and derive a TMNparameter mapped to the corresponding approximate reflectioncoefficient. In operation 314, the terminal may update the TMN parameterfor setting the load impedance of the antenna end. By updating the TMNparameter, lookup table search may be precisely performed to providehigh or maximum output power with respect to the updated position andthus a desired TMN parameter may be set.

In response to determining not to perform the precise lookup tablesearch (e.g., the calculated approximate reflection coefficient does notsatisfy the trigger condition), the terminal may optionally set to thedefault TMN mode. Setting to the default TMN mode may refer to settingto provide high (or maximum) output power with respect to a predefinedreference impedance position.

When the operation is completed up to operation 314, the terminal mayproceed to operation 300, returning to the standby mode, and repeatsubsequent operations.

FIG. 4 is a detailed flowchart illustrating a process of measuring anapproximate reflection coefficient for LUT-based real-time antennaimpedance matching according to an exemplary embodiment. FIG. 4illustrates a detailed example of a process described with reference toof FIG. 3.

There may be various types of implementation scenarios according toexemplary embodiments. Herein, a case where the bidirectional coupler isused is considered.

In operation 400, a terminal is in a standby state until its operationpoint is a CL-AIT operation point or more based on a trigger condition.The terminal may determine in operation 402 whether its operation pointis the CL-AIT operation point or more based on the trigger condition. Inother words, the terminal may determine whether to perform antennaimpedance matching, by determining whether it has measured the power ofthe signal transmitted through the antenna. When its operation point isthe CL-AIT operation point or more based on the trigger condition, theterminal may perform real-time monitoring on the bidirectional couplerin operation 404.

In operation 406, the terminal may set a TMN to a bypass mode. Here, thebypass mode may refer to setting a TMN parameter having the leastinsertion loss without the impedance change for the TMN. In operation408, the terminal may operate the bidirectional coupler through theforward path (or switch the bidirectional coupler to the forward path).In operation 410, the terminal may detect transmission and receptionsignals (e.g., a signal S_(fwd,tx)(t) and a signal S_(fwd,rx)(t+τ)), andstore the detected signals in the memory through the feedback loop.

Next, in operation 412, the terminal may operate the bidirectionalcoupler through the reverse path (or switch the bidirectional coupler tothe reverse path). In operation 414, the terminal may detecttransmission and reception signals (e.g., a signal S_(rev,tx)(t) and asignal S_(rev,rx)(t+τ)), and store the detected signals in the memorythrough the feedback loop.

In operation 416, the terminal may time-align the transmission andreception signals (e.g., signalsS_(fwd,tx)(t),S_(fwd,rx)(t+τ),S_(rev,tx)(t),S_(rev,rx)(t+τ)), calculateb4/b3 using these signals, and set the calculated b4/b3 as anapproximate reflection coefficient.

The terminal may determine in operation 418 whether a matrixdetermination condition for antenna impedance matching satisfies Case 1or Case 2.

When the matrix satisfies Case 1 in operation 418 (e.g., the approximatereflection coefficient does not satisfy the trigger condition), theterminal does not perform a precise TMN search, and may set to a defaultTMN value (e.g., TMN setting that ensures high or maximum transmissionpower), and then return the bidirectional coupler and enter the standbymode. Setting to the default TMN mode may refer to setting to providehigh or maximum output power with respect to a predefined referenceimpedance position.

On the other hand, when the matrix satisfies Case 2 in operation 418(e.g., the approximate reflection coefficient satisfies the triggercondition), the terminal may precisely search the lookup table toperform more precise impedance matching and derive a desired TMN settingvalue from the lookup table searching, in operation 420. In operation422, the terminal may update a TMN parameter. Updating the TMN parametermay refer to setting a TMN parameter to provide high or maximum outputpower with respect to the position updated from a predefined referenceimpedance position.

When operation 422 is completed, the terminal may return thebidirectional coupler in operation 424, and return to the standby mode.

FIGS. 5 and 6 are graphs illustrating the relationship between thereflection coefficients measured according to an exemplary the actualload impedance.

Referring to FIGS. 5 and 6, the relationship between the approximatereflection coefficients measured according to an exemplary embodimentand the actual load impedance is shown. More specifically, FIG. 5illustrates the actual load impedance, and FIG. 6 illustrates theapproximate reflection coefficients according to an exemplaryembodiment.

A reflection coefficient Γ, as shown in Equation (7) below, may berepresented as a magnitude |Γ| and a phase e^(jθ) ^(Γ) of the reflectioncoefficient Γ in the polar coordinate system, where the magnitude |Γ|has a range between 0 and 1, and the phase e^(jθ) ^(Γ) has a rangebetween −180° and +180°.┌=|┌|·e ^(jθ) ^(Γ) =┌_(g) _(e) +┌_(tm)=|┌| cos(θ_(Γ))+j·|┌|sin(θ_(Γ))  (7)

FIG. 5 illustrates load impedance setting values transferred through theload pull tuner. The load impedance setting values in FIG. 5 areobtained by adjusting the magnitude of the reflection coefficient from 0to 0.9 at intervals of 0.1, and the phase from −180° to 150° atintervals of 30°.

Generally, the magnitude of the reflection coefficient is a valueobtained by dividing, by the total radius, the distance from the centerpoint (e.g., the point indicating the characteristic impedance) to theload impedance point on the Smith chart, and the phase of the reflectioncoefficient refers to an angle up to the load impedance point.

FIG. 6 illustrates a b4/b3 matrix (e.g., approximate reflectioncoefficient) actually measured through the bidirectional coupler asdescribed above in FIG. 2, for each load impedance to be transferred,according to an exemplary embodiment. In FIG. 6, the b4/b3 value isobtained by adjusting the magnitude of the reflection coefficient from 0to 0.9 at intervals of 0.1, and the phase from −180° to 150° atintervals of 30°.

Although FIG. 6 shows magnitude scaling, phase rotation and center pointshift due to the approximation, it can be seen that one-to-one mappingis accurately performed for each load point.

Accordingly, FIG. 6 shows that the b4/b3 matrix is an effective matrixexpressing a reflection coefficient.

FIG. 7 is a graph illustrating the impedance matching coverage (orperformance) as per the number of reference impedance points to beincluded in the LUT for efficiency of the LUT size according to anexemplary embodiment.

Referring to FIG. 7, the x-axis represents VSWR of the load impedance,and the y-axis represents a cumulative distribution function (CDF) ofthe points satisfying the corresponding VSWR after undergoing impedancematching using the lookup table.

In an exemplary embodiment, to perform antenna impedance matching, theload impedance may be measured through measurement of the approximatereflection coefficient that is one-to-one mapped to the actualreflection coefficient without the measurement of the S-parameter forreal-time CL-AIT implementation, and a predefined lookup table may beprovided based thereon.

To reduce the additional complexity, the effective and smallest lookuptable size may be calculated in advance through the precise lookup tablesearch. The precise lookup table search represents a scheme forsearching the settable TMN values to select the best matching value.Assuming that a VSWR of 10:1 is defined as the upper limit, and theimpedance matching result as to the lookup table size is obtained bysetting a load impedance of random 10,000 points on the Smith chart, itis noted that in the case of long term evolution (LTE) low band, asshown in FIG. 7, the VSWR that is reduced more than 10:1 may bedetermined through a cumulative distribution function (CDF).

A method for determining the points to be used in generation of thelookup table may include a method for calculating a CDF satisfying thecorresponding VSWR for the entire impedance region and selecting thenumber of lookup table points that represent the desired CDF value. Inan exemplary embodiment, it is possible to generate the desired antennaimpedance using the load pull tuner during generation of the lookuptable, and estimate the approximate reflection coefficient therefor andthe TMN value transmitting high or maximum power, to thereby generatethe lookup table.

FIG. 8 is a graph illustrating a matching result before impedancematching is performed, a matching result obtained by using a 23-pointLUT, and a matching result obtained by calculating the accurate Γ_(L)through the precise lookup table search according to an exemplaryembodiment.

The x-axis represents a magnitude |Γ_(L)| of the load impedance and they-axis represents a VSWR of Γ_(in) after undergoing impedance matching.In a case where there is no matching impedance, as a magnitude of thereflection coefficient is greater, the VSWR increases more rapidly. Onthe other hand, when the impedance matching has been conducted, theconstant VSWR is maintained regardless of the magnitude of thereflection coefficient. Further, it can be seen in FIG. 8 that thematching result obtained by calculating the accurate Γ_(L) through theprecise lookup table search and the matching result obtained by usingonly the 23-point lookup table are similarly distributed within theerror range without a substantial difference, when viewed on the basisof a VSWR of 3:1. In other words, although the impedance matching isperformed using only the 23-point lookup table, the sufficientcorrection effect may be obtained.

Any constellation point may be used in the lookup table according to anexemplary embodiment. As an example, it is possible to use an effectivecoverage-increased lookup table by adjusting the 23-point lattice-likeload impedance structure in the form of 24-point radiation or polar toincrease or maximize the efficiency with the minimum lookup table. Thereference impedance of the lookup table may be continuously changed toincrease the efficiency.

FIGS. 9 and 10 are graphs illustrating distribution of the transmissionpower as per the load impedance before and after application of CL-AIT(the TMN bypass mode) according to an exemplary embodiment.

FIG. 9 illustrates the output power as per the load impedance in the TMNbypass mode (or TMN setting having the least TMN insertion loss).

FIG. 10 illustrates the load-specific output power after antennaimpedance matching by the AIT lookup table solution based on the sameload impedance according to an exemplary embodiment.

For comparison, FIG. 11 illustrates the power gain between FIG. 9(before application of the AIT lookup table solution) and FIG. 10 (afterapplication of the AIT lookup table solution).

In an area around the center having the low VSWR of the load impedance,the transmission power degradation by the reflected wave is low, andthus, the performance improvement may not be great. On the other hand,with respect to the load impedance point having the high VSWR, theperformance improvement may increase up to 3.5 dB˜4 dB. In other words,as the VSWR is higher, the performance improvement may be greater. Forexample, a VSWR of 10:1 represents the point where a reflectioncoefficient's magnitude |Γ_(L)| is 0.818.

The real-time CL-AIT scheme according to an exemplary embodiment mayperform real-time antenna impedance matching by estimating the actualload impedance through the measurement of the low-complexity approximatereflection coefficient. Further, the real-time CL-AIT scheme accordingto an exemplary embodiment may reduce or minimize the output power lossof the transmitter by estimating substantially all of the load impedancevariation through one-to-one mapping between the approximate reflectioncoefficient and the actual reflection coefficient.

In particular, in an exemplary embodiment, the reflection coefficientmay be measured based on the ratio of the signal reflected through thereverse path to the signal transmitted through the forward path of thebidirectional coupler, instead of measuring the reflection coefficientonly by using the signal reflected through the reverse path. Therefore,although the phases of elements (particularly, the elements of the RFunit, including the local oscillator) are initialized every time as theon/off or sleep mode operation is performed to reduce the powerconsumption, it is possible to calculate the accurate phase of thereflection coefficient at all times.

According to an exemplary embodiment, the call quality degradation bythe death grip phenomenon, in which the antenna sensitivity is loweredby, for example, a user's contact with the mobile device (e.g., holdingthe mobile device by a user's hand to make a call), occurring in themetallic designed mobile device may be avoided, and the propertransmission power may be maintained in real time despite the variouschanges to the load, thereby making it possible to increase the batterylife.

FIG. 12 is a graph illustrating an example of applying an antennaimpedance matching method according to another exemplary embodiment.

More specifically, FIG. 12 illustrates a phenomenon in which themeasured b4/b3 values are spread when there is a delay differencebetween the forward path and the reverse path.

Referring to FIG. 12, the phase change of b4/b3 is shown when a delayvalue is maintained at, for example, 20 in the forward path of thebidirectional coupler and a delay value is set to, for example, 17, 19,21 and 23 in the reverse path.

Assuming that the bidirectional coupler has four ports as shown in FIG.2, the impedance may be approximated as a ratio of a reflectedtransmission signal b4 incoming to the bidirectional coupler through thereverse path to a transmission output signal b3 incoming to thebidirectional coupler through the forward path.

A method of estimating the reflection coefficient through b4/b3 has beendescribed using Equations (1) to (5).

In reality, b4/b3 may be determined as a digital signal in the modem,and may be determined as a ratio of the maximum value of a correlationbetween a transmission signal s in the modem and a received signal rincoming through the bidirectional coupler, as shown in Equation (8)below.

$\begin{matrix}{\frac{b_{4}}{b_{3}} = {\frac{{{s_{1}(t)}}^{2}}{{{s_{2}(t)}}^{2}}\frac{\max\limits_{\tau_{2} \in D}\left( {{s_{2}(t)} \otimes {r_{2}\left( {t + \tau_{2}} \right)}} \right)}{\max\limits_{\tau_{1} \in D}\left( {{s_{1}(t)} \otimes {r_{1}\left( {t + \tau_{1}} \right)}} \right)}}} & (8)\end{matrix}$

In Equation (8), since delays τ₁, τ₂ include a delay on an analog frontpath of an antenna, every terminal has an error, and when the delays areset as a representative value, an error of b4/b3 may occur by the error.

Further, more errors may occur due to the delay variation as per thereal-time temperature change, and to solve this problem, there is a needfor a scheme capable of real-time delay estimation without an additionaldelay estimation device. In an exemplary embodiment, it is possible toestimate the delay in real time without the additional device, byestimating the analog front delay using the delay difference valuegenerated in the process of finding the maximum correlation value as inEquation (8).

Estimation for the actual delay may be represented as shown in Equation(9) below, and the delay may be measured for each of the forward pathand the reverse path.

$\begin{matrix}{D = \left\{ {{- \Delta},{{- \Delta} + 1},\ldots\mspace{14mu},0,\ldots\mspace{14mu},{{+ \Delta} - 1},{+ \Delta}} \right\}} & (9) \\{\Delta_{forward} = {\arg\;{\max\limits_{\tau_{1} \in D}\left\{ {{s_{1}\lbrack n\rbrack} \otimes {r_{1}\left\lbrack {n + \tau_{1}} \right\rbrack}} \right\}}}} & \; \\{\Delta_{reverse} = {\arg\;{\max\limits_{\tau_{2} \in D}\left\{ {{s_{2}\lbrack n\rbrack} \otimes {r_{2}\left\lbrack {n + \tau_{2}} \right\rbrack}} \right\}}}} & \; \\{{{{s\lbrack n\rbrack} \otimes {r\left\lbrack {n + \tau} \right\rbrack}} \equiv {\sum\limits_{n = 0}^{N - 1}{{s\lbrack n\rbrack}{r\left\lbrack {n + \tau} \right\rbrack}}}},} & \;\end{matrix}$

where Δ represents a delay value. In a case where the Δ value is large,when the convergence rate is high, the delay estimation may be quick,but the computation is large. On the other hand, in a case where the Δvalue is small, when the convergence rate is low, the delay estimationmay be slow, but the computation is small. To reduce the computation, itis set as D={−1, 0, +1}, and instead, the representative value measuredthrough calibration is entered as an initial delay value to reduce theconvergence time.

FIG. 13 is a block diagram of an antenna impedance matching apparatusaccording to another exemplary embodiment.

The antenna impedance matching apparatus in FIG. 13 may include a modem1300, and the modem 1300 may include a transmitter 1304, a data dumpunit 1306, a decoder, and a feedback receiver 1302.

A power amplifier 1320, a front-end module (FEM) 1330, a bidirectionalcoupler 1340, and a feedback receiver 1302 in FIG. 13 are similar to thepower amplifier PA, the duplexer 100, the bidirectional coupler 101 andthe feedback controller 102 in FIG. 1A, respectively, and detaileddescriptions thereof will be omitted.

The modem 1300 in FIG. 13 may include a data dump for correlationcalculation and the data dump unit 1306 that compensates for themeasured delay. As for the delay, the data dump unit 1306 in the modemmay compensate for the delay by adjusting the dump timing between thetransmitter 1304 and the feedback receiver 1302. Although not shown inthe drawing, the dump timing may be simply adjusted in the modem 1300 byregister setting.

By performing infinite impulse response (IIR) filtering on the delay ofthe maximum correlation value generated in the CL-AIT scheme andapplying the delay value calculated thereby back to the register, theinitial delay setting value may increase the convergence rate byapplying the representative value measured through calibration.

FIG. 14 is a flowchart illustrating an antenna impedance matching methodaccording to another exemplary embodiment. Specifically, FIG. 14illustrates a flowchart illustrating a process of estimating delay.

First, in operation 1400, the terminal may calculate a correlation valuefor each delay value, and select the maximum correlation value fromamong the calculated correlation values. Thereafter, in operation 1402,the terminal may select a delay value for the selected maximumcorrelation value.

In operation 1404, the terminal may calculate a bandwidth for theselected delay value, and select an IIR filter weight. This is to applya weight to an IIR filter input depending on the signal bandwidth. It ispossible to reduce the delay error in a narrow signal bandwidth byapplying a weight to an IIR filter input depending on the signalbandwidth during the real-time delay estimation. The signal bandwidthmay include a change to the bandwidth as per the communication systemscheme and/or the bandwidth as per the signal assignment.

In operation 1406, the terminal may perform IIR filtering on the delaymeasured in real time. During the next correlation measurement for thecalculated delay value, by using the calculated delay value as a delayvalue, it is possible to adaptively perform delay tracking.

A detailed operation of the IIR filtering is as follows.

In operation 1408, the terminal may determine whether an IIR filteringoutput value y[n] is greater than a threshold.

When the IIR filtering output value y[n] is greater than the threshold,the terminal may subtract 1 from the delay value, and subtract athreshold from the IIR filter buffer, in operation 1410.

On the contrary, when the IIR filtering output value y[n] is less than athreshold in operation 1414, the terminal may add 1 to the delay valueand add the threshold to the IIR filter buffer, in operation 1416.

When the IIR filtering output value y[n] is greater than the thresholdin operation 1414, the terminal may enter the bypass mode in operation1418.

Finally, after operation 1410, operation 1416 or operation 1418, theterminal may apply the delay value calculated through the IIR filteringas a delay setting value during the actual correlation measurement inoperation 1412. In other words, the terminal may use the calculateddelay value as a delay value during the next correlation measurement.

The terminal may compensate for the delay while the terminal maintainsto use the delay parameter value estimated through the IIR filteringprocess, for the register.

FIG. 15 is a diagram illustrating a fixed-point structure of an IIRfilter according to another exemplary embodiment.

FIG. 15 illustrates an example of applying a weight to an IIR filterinput. When a weight 1510 is applied to an IIR filter input based on thesignal bandwidth, a narrow-bandwidth signal may be introduced during thereal-time estimation. In this case, a delay error may occur due to theinaccurate correlation. To avoid the delay error, for example, a weight1510 of the IIR filter input may be set as 1 when it is greater than orequal to 12 resource blocks (RBs), and set as 0 when it is less than 12RBs. This is only an example and the exemplary embodiments are notlimited thereto.

The reason is as follows. As shown by reference numeral 1510, in ascenario of applying a weight to an IIR filter input depending on thesignal bandwidth, a narrow-bandwidth signal may come in during thereal-time estimation. In this case, a delay error may occur due to theinaccurate correlation.

In determining whether an IIR filtering output value y[n] is greaterthan a threshold (operation 1512), when the IIR filtering output valuey[n] is greater than the threshold, the terminal may subtract 1 from adelay offset, and subtract the threshold from an IIR filter buffer. Onthe contrary, when the IIR filtering output value y[n] is less than athreshold, the terminal may add 1 to a delay offset, and add thethreshold to the IIR filter buffer.

FIGS. 16A and 16B and FIGS. 17A and 17B are graphs illustrating thecorrelation result with the time-domain signal when the RB size is 1 and50 respectively, in LTE.

In the case of a 1-RB signal, the time-domain signal varies slowly andthe correlation also varies smoothly, and thus a delay variation (e.g.,dispersion) is large compared with a 50-RB signal. As a result, theremay occur a phenomenon in which the b4/b3 value fluctuates. Therefore,IIR filtering may be optionally performed when the signal bandwidth iswide, and a high weight may be applied to a wide-bandwidth signalcompared with a narrow-bandwidth signal, enabling robust impedancematching regardless of the signal bandwidth.

FIG. 18A is a graph illustrating forward and reverse real-time measureddelay values according to another exemplary embodiment.

FIG. 18A shows the delay estimation results when a terminal with theactual delay value=19 gives an error by applying 23 as an initial delayregister setting value for the forward path, and applying 22 for thereverse path. For example, for LTE Band 5 (the frequency bandwidth ofwhich is 844 Mhz for uplink (UL)) and 50 RB assignment, a delay 1 samplecorresponds to about 8 ns (or 61.44 Mhz sample rate).

FIG. 18B is a graph illustrating that delay register setting valuesconverge on 19, in a case where an IIR filter output value is greaterthan or equal to a threshold.

FIG. 18C is a graph illustrating the change in IIR filter outputs, andit can be seen that when the accumulated value exceeds a threshold, thethreshold is subtracted and the delay register setting value is reducedby 1.

When the delay is corrected and b4/b3 is measured in this way, it isshown that a constant value is represented as in FIG. 19. In FIG. 19,reference numeral 1910 represents the measured b4/b3, and the remainingpoints represent an LUT index of each load point, which is used forclosed-loop antenna impedance matching. When an IIR filter output is ina steady state by converging, it is possible to estimate a fractionaldelay with a ratio of the IIR filter output to the threshold by usingthis value. For example, when the threshold is 1200 and the IIR filteroutput value in the steady state is 23, the fractional delay is 1sample×23/1200 (or 8 ns×23/1200=0.15 ns). The fractional delay may becompensated in the dump delay control block 1306 in FIG. 13, and thepossible compensation resolution may vary depending on the hardwareresolution of the corresponding block. A phase offset of b4/b3, whichoccurs by the fractional delay, may be as shown in Equation (10) below.

$\begin{matrix}{\frac{b_{4}}{b_{3}} = {{\frac{{{s_{1}(t)}}^{2}}{{{s_{2}(t)}}^{2}} \cdot \frac{\left( {{s_{2}(t)} \otimes {r_{2}\left( {t + \tau_{2}} \right)}} \right)}{\left( {{s_{1}(t)} \otimes {r_{1}\left( {t + \tau_{1}} \right)}} \right)}}e^{i\; 2\;{\pi{({{f_{2}\tau_{2}} - {f_{1}\tau_{1}}})}}}}} & (10)\end{matrix}$

Herein, when τ₁, τ₂ is a fractional delay of the forward and reversepaths, a phase offset of e^(i2π(f) ² ^(τ) ² ^(−f) ¹ ^(τ) ¹ ⁾ may occur.Therefore, it is possible to directly compensate a phase offset value ofb4/b3 even for a phase offset below the hardware resolution.

The exemplary embodiments may increase the convergence rate by applyinga representative value measured through calibration as an initial delaysetting value by way of performing IIR filtering on a delay of themaximum correlation value occurring in the real-time closed-loop antennaimpedance matching method and applying the filtered delay value back tothe delay register.

The exemplary embodiments may reduce the delay error due to a narrowsignal bandwidth by applying a weight to an IIR filter input dependingon the signal bandwidth during the real-time delay estimation. Here, thesignal bandwidth may include a change to the bandwidth as per thecommunication system scheme and/or the bandwidth as per the signalassignment.

The exemplary embodiments may apply an IIR filter to each of abidirectional coupler for the forward path and a bidirectional couplerfor the reverse path during the real-time closed-loop antenna impedancematching, and apply the delay during the next correlation measurement,so the active real-time delay estimation may be possible, thereby makingit possible to reduce the delay measurement time occurring in themanufacturing process.

It should be noted that the drawings, flowcharts and block diagramsillustrated in FIGS. 1 to 19 are not intended to limit the scope. Inother words, not all of the information, fields, components oroperations disclosed in FIGS. 1 to 19 should be construed as theprerequisites for implementation, and only some of the components may beincluded to implement the disclosure without departing from the spiritand scope of the disclosure.

The above-described operations may be implemented by mounting a memorydevice storing the corresponding program code in the entity, thefunction, the base station and/or the terminal of the communicationsystem, and/or in any component of the vehicle device. In other words, acontroller of the entity, the function, the base station, the terminalor the vehicle device may run the above-described operations by readingthe program code stored in the memory device by means of a processor ora central processing unit (CPU).

Various components and modules of the entity, the function, the basestation, the terminal and/or the vehicle device described herein may beoperated using a hardware circuit (e.g., a complementary metal oxidesemiconductor (CMOS)-based logical circuit), or a hardware circuit suchas a combination of firmware, software and/or software inserted intohardware, firmware and/or a machine-readable medium. For example,various electrical structures and methods may be implemented usingelectrical circuits such as transistors, logic gates andapplication-specific integrated circuits (ASICs).

As is apparent from the foregoing description, according to theexemplary embodiments, it is possible to increase or maximize theefficiency of the transmission power of the antenna in the wirelesscommunication system.

Further, according to the exemplary embodiments, it is possible toincrease or maximize the transmission power of the antenna without thecomplex operation in the wireless communication system.

Moreover, according to the exemplary embodiments, it is possible tosubstantially prevent the death grip phenomenon in which thetransmission and/or reception sensitivity of the radio waves is abruptlylowered in the situation where the load impedance of the terminalvaries, in the wireless communication system.

While the disclosure has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the disclosure asdefined by the appended claims and their equivalents.

What is claimed is:
 1. An apparatus for matching antenna impedance in awireless communication system, the apparatus comprising: a controllerconfigured to: determine an approximate reflection coefficient based onan input signal and an output signal of a bidirectional couplerconnected to a signal path of an antenna, and determine an antennaimpedance matching parameter mapped to the determined approximatereflection coefficient in a lookup table; and an antenna impedancematching unit configured to perform antenna impedance matching based onthe antenna impedance matching parameter, wherein the input signal andthe output signal are propagated in both of a forward path and a reversepath of the bidirectional coupler, and wherein the controller isconfigured to apply infinite impulse response (IIR)-filtering in theforward path and the reverse path of the bidirectional coupler.
 2. Theapparatus of claim 1, wherein the approximate reflection coefficient isdetermined based on a ratio of output power of the bidirectional couplerin the reverse path to output power of the bidirectional coupler in theforward path.
 3. The apparatus of claim 1, wherein the input signalcomprises: a first forward path signal that is inputted to thebidirectional coupler in the forward path, and a first reverse pathsignal that is reflected from the antenna and inputted to thebidirectional coupler in the reverse path, and wherein the output signalcomprises: a second forward path signal that is outputted from thebidirectional coupler in the forward path, and a second reverse pathsignal that is outputted from the bidirectional coupler in the reversepath.
 4. The apparatus of claim 3, wherein the approximate reflectioncoefficient is determined as a ratio of a level of the second reversepath signal to a level of the second forward path signal.
 5. Theapparatus of claim 1, wherein the controller is further configured todetermine whether to perform the antenna impedance matching based onmeasured power of a signal transmitted through the antenna.
 6. Theapparatus of claim 1, wherein the controller is further configured todetermine whether the approximate reflection coefficient satisfies acertain condition, and in response to determining that the approximatereflection coefficient does not satisfy the certain condition, set adefault antenna impedance matching parameter corresponding to maximumtransmission power, and in response to determining that the approximatereflection coefficient does not satisfy the certain condition, searchfor an additional antenna impedance matching parameter.
 7. The apparatusof claim 1, wherein the controller is further configured to obtain anerror of the approximate reflection coefficient due to a delay on ananalog front path.
 8. The apparatus of claim 7, wherein the controlleris further configured to obtain the error of the approximate reflectioncoefficient by performing: obtaining a correlation value of a receivedsignal with respect to a transmission signal; selecting a maximumcorrelation value from among the obtained correlation value; selecting adelay value corresponding to the maximum correlation value; performingthe IIR-filtering on the selected delay value; and applying the filtereddelay value to obtain a next correlation value.
 9. The apparatus ofclaim 1, wherein the controller is further configured to generate thelookup table by determining a cumulative distribution function (CDF)that satisfies a voltage standing wave ratio (VSWR) with respect to anentire antenna impedance region, and selecting a number of points in thelookup table, based on the CDF value.
 10. The apparatus of claim 1,wherein the controller is further configured to generate a loadimpedance by using a load pull tuner, and generate the lookup table byestimating the approximate reflection coefficient based on the loadimpedance, and an antenna impedance matching parameter that correspondsto maximum transmission power.
 11. The apparatus of claim 8, wherein thecontroller is configured to compensate for the delay by applying thefiltered delay value to a delay register.
 12. The apparatus of claim 8,wherein the controller is configured to obtain a signal bandwidthcorresponding to the delay value corresponding to the maximumcorrelation value, and select an IIR filter weight based on the signalbandwidth.
 13. The apparatus of claim 12, wherein the signal bandwidthcomprises a change to at least one of a bandwidth as per a communicationsystem scheme and a bandwidth as per signal assignment.
 14. An apparatusfor matching antenna impedance in a wireless communication system, theapparatus comprising: a bidirectional coupler connected to a signal pathof an antenna; and a controller configured to: determine an approximatereflection coefficient based on output power of the bidirectionalcoupler in a reverse path and output power of the bidirectional couplerin a forward path of the bidirectional coupler, and determine an antennaimpedance matching parameter corresponding to the determined approximatereflection coefficient by using a lookup table, and perform antennaimpedance matching based on the approximate reflection coefficient,wherein the controller is configured to apply infinite impulse response(IIR)-filtering in the forward path and the reverse path of thebidirectional coupler.
 15. The apparatus of claim 14, wherein thecontroller is further configured to determine the approximate reflectioncoefficient based on a ratio of the output power of the bidirectionalcoupler in the reverse path to the output power of the bidirectionalcoupler in the forward path.
 16. An apparatus for matching antennaimpedance in a wireless communication system, the apparatus comprising:a bidirectional coupler connected to a signal path of an antenna; atleast one tunable matching network (TMN) including at least one of animpedance tuner or an aperture tuner; and at least one controllerconfigured to: determine an approximate reflection coefficient based onan input signal and an output signal of the bidirectional coupler,determine at least one antenna impedance matching parameter mapped tothe determined approximate reflection coefficient in a lookup table, andperform antenna impedance matching by controlling the at least one TMN,based on the determined at least one antenna impedance matchingparameter, wherein the input signal and the output signal are propagatedin both of a forward path and a reverse path of the bidirectionalcoupler, and wherein the at least one controller is configured to applyinfinite impulse response (IIR)-filtering in the forward path and thereverse path of the bidirectional coupler.
 17. The apparatus of claim16, wherein the at least one controller is further configured to obtainan error of the approximate reflection coefficient by performing:obtaining a correlation value of a received signal with respect to atransmission signal; selecting a maximum correlation value from amongthe obtained correlation value; selecting a delay value corresponding tothe maximum correlation value; performing the IIR-filtering on theselected delay value; and applying the filtered delay value to obtain anext correlation value.
 18. The apparatus of claim 17, wherein the atleast one controller is further configured to obtain a signal bandwidthcorresponding to the delay value corresponding to the maximumcorrelation value, and select an IIR filter weight based on the signalbandwidth.
 19. The apparatus of claim 16, wherein the at least onecontroller is further configured to obtain the lookup table byestimating approximate reflection coefficients corresponding tovariation of a load impedance that is generated by using a load pulltuner.